The process involves the dosing, controlled by speed of rotation, of the thermal output of a combined heat and power generation system, especially a block heating and power station.
Block heating and power stations (BHPS) are relatively small power plants for the production of electric current and heat, the electric current being produced by means of generators driven by heat engines (e.g. reciprocating engines, Wankel engines, Stirling engines or gas turbines). Heat is produced by utilizing the lost heat from cooling water and exhaust gas. This type of energy production is also referred to as combined heat and power (CHP).
On the one hand, previous BHPS were operated with a constant speed of rotation (e.g. 1500 rpm) in order to produce and maintain the required mains frequency (e.g. 50 Hz) via a generator. On the other hand, present-day BHPS are normally operated under full load, i.e. with the throttle fully open, because this maximizes the efficiency for a given speed of rotation.
These two demands for constant speed of rotation and maximum load prevent the present-day plants from being able to be adapted to a change in the heat requirement, e.g. of a connected domestic heating system. It is known that the output of heat engines can only be varied by changing either the speed of rotation or the load. If, for example, an attempt were made to reduce the lost heat output by closing the throttle by a certain amount while maintaining a constant speed of rotation, this measure would only be partially successful. Although the generator output would fall as a function of the throttle position due to the drop in torque on the shaft, the heat emission would only be inadequately reduced because, in a throttled heat engine, the efficiency deteriorates as a result of the throttle losses and the falling average pressure of the heat engine.
Not uncommonly, therefore, BHPS consist of several heat engine generator units and a heating boiler for peak outputs, so that individual units can be switched on or off in order to adjust the power emission of the BHPS as closely as possible to the annual profile (i.e. the cumulative frequency distribution of the thermal output requirement of a consumer unit over a one-year period).
This type of annual profile, such as that illustrated for a particular housing unit in FIG. 4, clearly shows that the thermal output requirement, taken over the year, can be very variable. When planning a BHPS, the annual profile can be used to apportion the capacity to be installed among heat engine generator units and peak load boilers. In FIG. 4, for example, 5 heat engine generator units (M1-M5) are provided for a total of 500% of the maximum output and one conventional heating boiler (hatched area) is provided for the remaining 50% to cover peak outputs, so as to be able to produce the thermal output (Q) of 100% which is required over the year. The rectangles drawn in for the partial outputs indicate the annual work or hours under full load (0-8760 h/a=heating hours per year). It can be seen that the individual engine in FIG. 4 can only cover ca. 10% of the total output. With favourable annual profiles, this percentage can be increased to 15% according to the state of the art.
FIG. 5 illustrates another annual profile for a conventional BHPS with one heat engine generator unit which can produce 30% of the thermal output (Q) (hatched area). For the two non-hatched areas, a conventional heating system must additionally be available in order to supplement the BHPS when the heat requirement is &gt;30% and take over the heat production on its own when the heat requirement is &lt;30% and the BETS is switched off
Surprisingly, it has now been found that the dosing of the thermal output, while maintaining the position of the load controls (e.g. throttle) in the region of maximum efficiency of the heat engine, can be effected by changing the speed of rotation of the heat engine or the generator by discharging more or less electric power, preferably into the public supply network.
Accordingly, a combined heat and power generation system operated by means of the process of the invention, especially a BHPS, no longer suffers from the above-mentioned problems because the power output of such a system, even with only one heat engine generator unit, can always be adapted to the annual profile by controlling the heat emission.
This is achieved by a procedure whereby the heat engine is brought to the full load position (open throttle) or optimum efficiency simply by reducing or increasing the speed of rotation to a lower or higher level by changing the discharge of electricity, e.g. into the public supply network.
If the generator is forced via a current regulator to discharge more electricity into the public supply network, its load increases and the heat engine generator unit is thus slowed down to a lower speed of rotation. This is how the output can be adjusted to any desired level while always keeping the throttle open. Current regulation corresponds to the usual method by which a voltage increase above the level of mains voltage also results in an increased current flow.
The dosing of the heat emission of a BHPS under full load can thus be effected by loading of the generator with the mains supply and consequent regulation of the speed of rotation.
Because changes in speed of rotation of the engine also change the frequency of the electric current it produces, which is undesirable as the consumer requires a uniform alternating current frequency of 50 Hz, the electric current can be kept at the mains frequency e.g. by conventional AC/DC/AC transformation using a frequency converter.
The process according to the invention and a device for carrying it out are illustrated in greater detail below.